With approximately 14,000 new infections per day, there is an urgent need to develop a safe and effective HIV vaccine. However, all of the strategies tested to date have fallen short of this goal. Thus, new strategies are required if the development of a safe and effective vaccine to prevent HIV infection is ever to be realized. A recent HIV Vaccine Summit held at NIH in March 2008 listed nine high priority research objectives essential for the development of an HIV vaccine. The work proposed in this grant directly addresses one of the nine highest research priorities listed, namely: Determine why broadly neutralizing antibodies (bNAbs) are uncommon and how they can be elicited. Our new approach to this problem is based on surprising preliminary results showing that cleavage sites on HIV gp120, recognized by intracellular and secreted enzymes known to be important for antigen processing, tumor growth, and parasitic infection, are highly conserved. Surprisingly, these sites are also located at, or adjacent to, specific amino acids important for receptor binding or the binding of neutralizing antibodies. Because of the high rate of virus variation, these sites must have been conserved as the result of strong selective pressure, possibly to evade the immune response. In this proposal we wish to inactivate these conserved cleavage sites on HIV envelope proteins to create protease resistant envelope glycoproteins. We will then express these in quantities required for rabbit immunogenicity studies. The resulting sera will be assayed for the presence of broadly neutralizing antibodies. We postulate that inactivation of these sites will preserve important epitopes that are normally degraded in vivo before they are able to stimulate robust immune responses. Preservation of these epitopes should allow us to redirect the immune response to HIV envelope proteins in a way that improves the formation of broadly neutralizing antibodies. These experiments have the potential to explain why it has been so difficult to elicit neutralizing antibodies with the vaccines tested to date, as well as other observations that have perplexed the field for many years.